Pd 2L2 Research Articles

Engineering Soluble Diketopyrrolopyrrole Chromophore Stacks from a Series of Pd(II)-Based Ravels.

A strategy to engineer the stacking of diketopyrrolopyrrole (DPP) dyes based on non-statistical metallosupramolecular self-assembly is introduced. For this, the DPP backbone is equipped with nitrogen-based donors that allow for different discrete assemblies to be formed upon the addition of Pd(II), distinguished by the number of π-stacked chromophores. A Pd3 L6 three-ring, a heteroleptic Pd2 L2 L'2 ravel composed of two crossing DPPs (flanked by two carbazoles), and two unprecedented self-penetrated motifs (a Pd2 L3 triple and a Pd2 L4 quadruple stack), were obtained and systematically investigated. With increasing counts of stacked chromophores, UV/Vis absorptions red-shift and emission intensities decrease, except for compound Pd2 L2 L'2 , which stands out with an exceptional photoluminescence quantum yield of 51 %. This is extraordinary for open-shell metal containing assemblies and explainable by an intra-assembly FRET process. The modular design and synthesis of soluble multi-chromophore building blocks offers the potential for the preparation of nanodevices and materials with applications in sensing, photo-redox catalysis and optics.

A Template-Free Pd2 L4 Cage with up to Nanomolar Affinity for Chloride in Aqueous Solutions.

Invited for the cover of this issue is the group of Dillip Kumar Chand at the Indian Institute of Technology Madras. The image depicts the efficient chloride binding ability of a designer self-assembled coordination cage in water. The binding occurs selectively even in presence of models of biologically abundant anionic systems. Read the full text of the article at 10.1002/chem.202300891.

A Template-Free Pd2 L4 Cage with up to Nanomolar Affinity for Chloride in Aqueous Solutions.

Selective binding of chloride over the other most abundant anions in living organisms is pivotal due to its essential role in physiological functions. Herein, we report a template-free Pd2 L4 cage exhibiting high selectivity for medium-sized halides (i. e., Cl- , Br- ) in water owing to the size-discriminatory nature of the cage cavity. In pure water, this cage displays high selectivity and micromolar affinity for chloride. The cage shows no binding towards other biologically more abundant essential anions such as phosphates, carboxylates, or bicarbonate. This cage shows an unprecedented nanomolar affinity with 1 : 1 binding stoichiometry for chloride in aqueous-DMSO media. This high affinity was achieved with the best use of traditional hydrogen bonding and electrostatic interactions, as confirmed by single-crystal X-ray diffraction analysis. This well-defined cage sequestrates F- by cleaving a B-F bond in BF4 - in a facile manner in a nonpolar solvent or in the presence of excess ligand. This cage also demonstrates capture of the sub-ppm chloride level that is present in commercial D2 O samples.

Tailor-Made Pdn L2n Metal-Organic Cages through Covalent Post-Synthetic Modification.

Post-synthetic modification (PSM) is an effective approach for the tailored functionalization of metal-organic architectures, but its generalizability remains challenging. Herein we report a general covalent PSM strategy to functionalize Pdn L2n metal-organic cages (MOCs, n=2, 12) through an efficient Diels-Alder cycloaddition between peripheral anthracene substituents and various functional motifs bearing a maleimide group. As expected, the solubility of functionalized Pd12 L24 in common solvents can be greatly improved. Interestingly, concentration-dependent circular dichroism and aggregation-induced emission are achieved with chiral binaphthol (BINOL)- and tetraphenylethylene-modified Pd12 L24 , respectively. Furthermore, Pd12 L24 can be introduced with two different functional groups (e.g., chiral BINOL and achiral pyrene) through a step-by-step PSM route to obtain chirality-induced circularly polarized luminescence. Moreover, similar results are readily observed with a smaller Pd2 L4 system.

Controlled Construction of Heteroleptic [Pd2 (LA )2 (LB )(LC )]4+ Cages: A Facile Approach for Site-Selective endo-Functionalization of Supramolecular Cavities.

Construction of supramolecular structures with internal functionalities is a promising approach to build enzyme-like cavities. The endo-functionalized [Pd12 L24 ] and [Pd2 L4 ] coordination cages represent the most successful systems in this regard. However, these systems mainly contain one type of endo-moiety. We herein provide a solution for the controlled endo-functionalization of [Pd2 L4 ] cages. Site-selective introduction of the endo-functional group was achieved through the formation of heteroleptic [Pd2 (LA )2 (LB )(LC )] cages. Using two orthogonal steric control elements is the key for the selective formation of the hetero-assemblies. We demonstrated the construction of two hetero-cages with a single internal functional group as well as a hetero-cage with two distinct endohedral functionalities. The endo-functionalized hetero-cages bound sulfonate guests with fast-exchange dynamics. This strategy provides a new solution for the controlled endo-functionalization of supramolecular cavities.

Anion-Driven Programmable Chiral Self-Sorting in Metal-Organic Cages and Structural Transformations between Heterochiral and Homochiral Cages.

When a racemic mixture of chiral building blocks self-assembles to form discrete molecular or supramolecular cages, the system can adopt either social or narcissistic chiral self-sorting. However, control over such chiral self-sorting is hard to achieve with a desired choice of outcome. Herein, we report anion templated high-fidelity chiral self-sorting during the coordination-driven self-assembly of [Pd2 L4 ] metal-organic cages, with a racemic mixture of an axially chiral ligand. Upon varying the counter-anions, the outcome of the choice of chiral self-sorting, whether social or narcissistic, leading to kinetically favored heterochiral or thermodynamically favored homochiral cages, can be controlled through specific anion encapsulation. Non-encapsulating anion afforded a mixture of all possible diastereomers. Anion exchange enabled structural transformations between the diastereomers and the conversion of the mixture of diastereomers into homochiral diastereomers.

PET Imaging of Self-Assembled 18 F-Labelled Pd2 L4 Metallacages for Anticancer Drug Delivery.

To advance the design of self-assembled metallosupramolecular architectures as new generation theranostic agents, the synthesis of 18 F-labelled [Pd2 L4 ]4+ metallacages is reported. Different spectroscopic and bio-analytical methods support the formation of the host-guest cage-cisplatin complex. The biodistribution profiles of one of the cages, alone or encapsulating cisplatin have been studied by PET/CT imaging in healthy mice in vivo, in combination to ICP-MS ex vivo.

High-Throughput Computational Evaluation of Low Symmetry Pd2 L4 Cages to Aid in System Design*.

Unsymmetrical ditopic ligands can self‐assemble into reduced‐symmetry Pd2L4 metallo‐cages with anisotropic cavities, with implications for high specificity and affinity guest‐binding. Mixtures of cage isomers can form, however, resulting in undesirable system heterogeneity. It is paramount to be able to design components that preferentially form a single isomer. Previous data suggested that computational methods could predict with reasonable accuracy whether unsymmetrical ligands would preferentially self‐assemble into single cage isomers under constraints of geometrical mismatch. We successfully apply a collaborative computational and experimental workflow to mitigate costly trial‐and‐error synthetic approaches. Our rapid computational workflow constructs unsymmetrical ligands and their Pd2L4 cage isomers, ranking the likelihood for exclusively forming cis‐Pd2L4 assemblies. From this narrowed search space, we successfully synthesised four new, low‐symmetry, cis‐Pd2L4 cages.

A Robust Photocatalytic Hybrid Material Composed of Metal-Organic Cages and TiO2 for Efficient Visible-Light-Driven Hydrogen Evolution.

The design of photochemical molecular devices (PMDs) for photocatalytic H2 production from water is a meaningful but challenging subject currently. Herein, a Pd2 L4 type metal-organic cage (denoted as MOC-Q2) is designed as a PMD, which consists of two catalytic centers (Pd2+ ) and four photosensitive ligands (L-2) with four pyridine anchoring groups. Subsequently, the MOC-Q2 is combined with TiO2 to form TiO2 -MOC-Q2 hybrid materials with different MOC-Q2 contents by a facile sol-gel method, which have micro/mesoporous structures and large surface areas. The optimized TiO2 -MOC-Q2 (6.5 wt%) exhibits high H2 production activity (7.9 mmol g-1 h-1 within 5 h) and excellent durability, giving a TON value of 23477 or 11739 (based on MOC-Q2 or Pd moles) after recycling for 7 rounds. By contrast, the pure MOC-Q2 only shows an ordinary photocatalytic H2 production rate (0.84 mmol g-1 h-1 within 5 h) in the homogeneous system. It can be deduced that TiO2 drives the photocatalysis and simultaneously acts as the structure promoter. This study presents a meaningful and distinctive attempt of a new approach for the design and development of MOC-based heterogeneous photocatalysts.

Dynamic Open Coordination Cage from Nonsymmetrical Imidazole-Pyridine Ditopic Ligands for Turn-On/Off Anion Binding.

This work demonstrates a new nonconventional ligand design, imidazole/pyridine-based nonsymmetrical ditopic ligands (1 and 1S ), to construct a dynamic open coordination cage from nonsymmetrical building blocks. Upon complex formation with Pd2+ at a 1:4 molar ratio, 1 and 1S initially form mononuclear PdL4 complexes (Pd2+ (1)4 and Pd2+ (1S )4 ) without formation of a cage. The PdL4 complexes undergo a stoichiometrically controlled structural transition to Pd2 L4 open cages ((Pd2+ )2 (1)4 and (Pd2+ )2 (1S )4 ) capable of anion binding, leading to turn-on anion binding. The structural transitions between the Pd2 L4 open cage and the PdL4 complex are reversible. Thus, stoichiometric addition (2 equiv) of free 1S to the (Pd2+ )2 (1S )4 open cage holding a guest anion ((Pd2+ )2 (1S )4 ⋅G- ) enables the structural transition to the Pd2+ (1S )4 complex, which does not have a cage and thus causes the release of the guest anion (Pd2+ (1S )4 +G- ).

Self-assembly and Cycling of a Three-state Pdx Ly Metallosupramolecular System.

The use of stimuli to induce reversible structural transformations in metallosupramolecular systems is of keen interest to chemists seeking to mimic the way that Nature effects conformational changes in biological machinery. While a wide array of stimuli have been deployed towards this end, stoichiometric changes have only been explored in a handful of examples. Furthermore, switching has generally been between only two distinct states. Here we use a simple 2-(1-(pyridine-4-methyl)-1H-1,2,3-triazol-4-yl)pyridine "click" ligand in combination with PdII in various stoichiometries and concentrations to quantitatively access and cycle between three distinct species: a [PdL2 ]2+ monomer, a [Pd2 L2 ]4+ dimer, and a [Pd9 L12 ]18+ cage.

Reversible Transformation between a [PdL2 ]2+ "Figure-of-Eight" Complex and a [Pd2 L2 ]4+ Dimer: Switching On and Off Self-Recognition.

Structural changes to metallosupramolecular assemblies resulting in the release or uptake of guests are currently well established, whereas transformations turning on and off specific self-recognition are far less developed. We report a novel ligand (2,6-bis(1-(3-pyridin-4-yl)phenyl-1H-1,2,3-triazol-4-yl)pyridine) possessing a tridentate central metal-binding site flanked by two pendant pyridyl arms. In a 2:1 ratio with PdII metal ions, a spiro-type [PdL2 ]2+ "Figure-of-eight" complex forms with the central tridentate binding pocket unoccupied. The introduction of an additional one equivalent of PdII metal ion results in the conversion to a dimeric [Pd2 L2 ]4+ molecule with the tridentate pocket occupied. There is site-specific self-recognition between dimers in solution with strong NOE peaks between adjacent molecules. The self-recognition between dimers can be turned off in two ways: firstly, adding another equivalent of PdII metal ion brings about binding to the previously uncoordinated pyridyl arms that are key to the self-recognition event, and; secondly, addition of sufficient ligand to return the stoichiometry to 2:1 regenerates the [PdL2 ]2+ complex. Hence, the self-recognition event can be turned on or off through simple variation of L:PdII stoichiometry.

Discrete and Polymeric Self-Assembled Palladium(II) Complexes as Supramolecular Gelators.

Supramolecular gels prepared from low-molecular-weight gelators have been extensively explored. However, the exploitation of discrete or polymeric metal complexes as gelators is a relatively recent trend. The synthesis of self-assembled coordination complexes from palladium(II) and selected ligands is well established, but the potential of these complexes as gelators is a less explored treasure. Herein we focus on the gelation abilities of some self-assembled palladium(II) complexes and the resulting unique properties. First, discrete complexes with PdL, PdL2 , Pd2 L, Pd2 L2 , Pd2 L4 , and Pd3 L6 compositions are discussed. Second, gelation behavior promoted by coordination-polymer-like gelators formed in situ is explored. These gel samples have been employed in catalysis and the uptake of organic and dye molecules from the solution and gas phases. It is concluded that untapped unique properties can be realized by further exploration of designer palladium(II) complexes.

Construction of Metallacage-Cored Supramolecular Gel by Hierarchical Self-Assembly of Metal Coordination and Pillar[5]arene-Based Host-Guest Recognition.

In this work, a Pd2 L4 metallacage 2•([BF4 ]- )4 with four pillar[5]arene units is first prepared and characterized by 1D multinuclear NMR (1 H, 11 B, and 19 F NMR), 2D 1 H-1 H correlation spectra, 1 H-13 C heteronuclear single quantum coherence, and diffusion-ordered NMR spectroscopy, and electrospray ionization time-of-flight mass spectrometry. By the introduction of a ditopic guest molecule 3 into a chloroform solution of 2•([BF4 ]- )4 , a supramolecular polymer network gel is successfully constructed based on the metal coordination interactions and host-guest recognition between the pillar[5]arene units of 2•([BF4 ]- )4 and neutral ditopic guest molecule 3. The temperature and pH responsivenesses of the supramolecular gel are studied, which are further employed for the controlled release of different cargos. As a demonstration, emodin and methylene blue are trapped in the cavities of the metallacage and in the pores of the supramolecular gel, respectively. Methylene blue is first released along with the gel-sol transition while emodin is then released by the further addition of acid to destroy the metallacage. This study explores the use of metallacage-cored supramolecular network gels for sequential controlled release and contributes to the development of smart and adaptive materials.

A Rotaxane‐like Cage‐in‐Ring Structural Motif for a Metallosupramolecular Pd6L12 Aggregate

A BODIPY-based bis(3-pyridyl) ligand undergoes self-assembly upon coordination to tetravalent palladium(II) cations to form a Pd6 L12 metallosupramolecular assembly with an unprecedented structural motif that resembles a rotaxane-like cage-in-ring arrangement. In this assembly the ligand adopts two different conformations-a C-shaped one to form a Pd2 L4 cage which is located in the center of a Pd4 L8 ring consisting of ligands in a W-shaped conformation. This assembly is not mechanically interlocked in the sense of catenation but it is stabilized only by attractive π-stacking between the peripheral BODIPY chromophores and the ligands' skeleton as well as attractive van der Waals interactions between the long alkoxy chains. As a result, the co-arrangement of the two components leads to a very efficient space filling. The overall structure can be described as a rotaxane-like assembly with a metallosupramolecular cage forming the axle in a metallosupramolecular ring. This unique structural motif could be characterized via ESI mass spectrometry, NMR spectroscopy, and X-ray crystallography.

Donor-Site-Directed Rational Assembly of Heteroleptic cis-[Pd2 L2 L'2 ] Coordination Cages from Picolyl Ligands.

A donor‐site engineering approach facilitates the formation of heteroleptic [Pd2 L 2 L′2]4+ cage structures through a favored cis‐′in2/out2′ spatial configuration of the methyl groups of 5‐ and 3‐substituted bis‐monodentate picolyl ligands with flat acridone and bent phenothiazine backbones. The heteroleptic cages were confirmed by ESI‐MS and 2D NMR experiments as well as DFT calculations, which pointed toward a cis‐configuration being energetically favored. This was further supported by the synthesis and X‐ray structure of a previously unreported cis‐[Pd(2‐picoline)4]2+ complex. The formation of homoleptic structures, however, was met with considerable steric hindrance at the PdII centers, as observed by the formation of [Pd2 L 3(solvent)2]4+ and [Pd2 L 2(solvent)4]4+ species when only one type of acridone‐based ligand was offered. In contrast, bent phenothiazine ligands with outside‐pointing methyl groups showed the ability to form interpenetrated double‐cages, as revealed by X‐ray crystallography. The general route presented herein enables the assembly of uniform cis‐[Pd2 L 2 L′2]4+ coordination cages, thus furthering the possibility to increase structural and functional complexity in supramolecular systems.

Energy-Landscape-Independent Kinetic Trap of an Incomplete Cage in the Self-Assembly of a Pd2 L4 Cage.

A kinetic trap is the metastable species that is transiently or constantly produced during the reaction by trapping in a deep energy well. In most cases, the reactivity of kinetically trapped species is relatively low under the reaction conditions. Herein, we report another type of kinetically trapped species that is an incomplete cage (IC) intermediate produced during the self-assembly of a Pd2 L4 cage from ditopic ligand (L) and PdII ions with a certain lifetime, although IC has a high enough reactivity to be converted into the cage with the reaction of free L, which was confirmed by the reaction of the isolated IC and L under the self-assembly conditions. IC was kinetically trapped not because IC lies on the bottom of a deep energy well but because the conversion of the intermediates essential for the conversion of IC to the cage preferentially takes place; IC was kinetically trapped independently of the shape of the energy landscape of the self-assembly.

Cover Feature: Energy‐Landscape‐Independent Kinetic Trap of an Incomplete Cage in the Self‐Assembly of a Pd2L4 Cage (Chem. Eur. J. 35/2018)

Cover Feature: Energy‐Landscape‐Independent Kinetic Trap of an Incomplete Cage in the Self‐Assembly of a Pd₂L₄ Cage (Chem. Eur. J. 35/2018)

Programmable Self-Assembly of Heterometallic Palladium(II)-Copper(II) 1D Grid-Chain using Dinuclear Palladium(II) Corners with Pyrazole-Carboxylic Acid Ligands.

A novel heterometallic diPdII -diCuII grid-chain, {[(bpy)4 Pd4 Cu2 L4 ](NO3 )4 }n (2; bpy=2,2'-bipyridine), was synthesized through a programmable self-assembly approach from the molecular corners [(bpy)2 Pd2 (HL)(L)](NO3 ) (1) as linkers with CuII nitrate by using the bifunctional H2 L ligand featuring primary (pyrazole) and secondary (benzoic acid) groups. Structural analysis revealed that 1D structure 2 consists of one [Cu2 (O2 CPh)4 ]n unit as a bridge and two [(bpy)2 Pd2 L2 ]n corners. Additionally, the catalytic effect of the heterometallic synergy on the Suzuki coupling reaction by using 2 was further explored.

Self-Assembly Process of a Pd2 L4 Capsule: Steric Interactions between Neighboring Components Favor the Formation of Large Intermediates.

The effect of molecular interactions between the components on the self-assembly process of Pd2 L4 structures was investigated by a 1 H NMR-based quantitative approach (QASAP: quantitative analysis of self-assembly process). Although the self-assembly of the Pd2 L4 cage without interactions between the bent ligands took place, mainly producing small intermediates, the self-assembly of the Pd2 L4 capsule composed of bent ligands with anthracene panels tends to produce large intermediates containing more components than the capsule. This is ascribed to steric interactions between the panels.